New Token class to hold information about a value to test

It holds the type of the token, its value, an id to identify it in the
session and an optional index if it is part of a sequence.
New class to create a template of tokens, especially for SQLi tests.

It basically defines a type and holds a ordered list of elements in
order to generate a list of sequenced tests, mainly for SQL injection
detection for the moment.
New class TokenGenerator to dissociate the generation from the tests.

This class generates a list of tokens to be used for tests. It uses an
unique Id (an int) for each generated token during one session, to
identify it easily during the tests.
The ConfigurationHolder is a base for a central place for configuration
of this plugin.

It will handle runtime values (like if SQLi tests must be performed or
not) and values retrieved from the storage, to ensure persistent
configuration and centralized in one class.
Simple class to permit some tests. Might be deleted in the future.
Taking into account new token generation.